The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient""s body lumen, such as a blood vessel, to maintain the patency thereof, along with systems for delivering and deploying such stents. Stents are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy or other means, to help improve the results of the procedure and reduce the possibility of restenosis.
Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other arterial lumen, such as a coronary artery. Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position. They are particularly suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway there through.
A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from an expandable heat sensitive metal; and self-expanding stents inserted into a compressed state for deployment into a body lumen. One of the difficulties encountered in using prior art stents involve maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery and accommodate the often tortuous path of the body lumen.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed from, for example, shape memory metals or superelastic nickel-titanum (NiTi) alloys, which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery catheter into the blood vessel. Such stents manufactured from expandable heat sensitive materials allow for phase transformations of the material to occur, resulting in the expansion and contraction of the stent. Other stents include those made with a branded configuration that does not go through plastic deformation.
Details of prior art expandable stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,1338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maass, et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 5,514,154 (Lau, et al.); U.S. Pat. No. 5,421,955 (Lau et al.); U.S. Pat. No. 5,603,721 (Lau et al.); U.S. Pat. No. 4,655,772 (Wallsten); U.S. Pat. No. 4,739,762 (Palmaz); and U.S. Pat. No. 5,569,295 (Lam), which are hereby incorporated by reference.
Further details of prior art self-expanding stents can be found in U.S. Pat. No. 4,580,568 (Gianturco); and U.S. Pat. No. 4,830,003 (Wolff, et al.), which are hereby incorporated by reference.
Some prior art stent delivery systems for implanting self-expanding stents include an inner lumen upon which the compressed or collapsed stent is mounted and an outer restraining sheath which is initially placed over the compressed stent prior to deployment. When the stent is to be deployed in the body vessel, the outer sheath is moved in relation to the inner lumen to xe2x80x9cuncoverxe2x80x9d the compressed stent, allowing the stent to move to its expanded condition. Some delivery systems utilize a xe2x80x9cpush-pullxe2x80x9d type technique in which the outer sheath is retracted while the inner lumen is pushed forward. Still other systems use an actuating wire which is attached to the outer sheath. When the actuating wire is pulled to retract the outer sheath and deploy the stent, the inner lumen must remain stationary, preventing the stent from moving axially within the body vessel.
However, problems have been associated with prior art delivery systems. For example, systems which rely on a xe2x80x9cpush-pull designxe2x80x9d can experience movement of the collapsed stent within the body vessel when the inner lumen is pushed forward which can lead to inaccurate positioning and, in some instances, possible perforation of the vessel wall by a protruding end of the stent. Systems which utilize an actuating wire design will tend to move to follow the radius of curvature when placed in curved anatomy of the patient. As the wire is actuated, tension in the delivery system can cause the system to straighten. As the system straightens, the position of the stent changes because the length of the catheter no longer conforms to the curvature of the anatomy. This change of the geometry of the system within the anatomy can also lead to inaccurate stent positioning.
Another difficulty which can be encountered with some existing self-expanding stents is the fact that the length of the stent can shorten dramatically during deployment, making it difficult to precisely position the stent within the artery. Since proper positioning of the stent is critical to the performance of the stent, it is imperative that the physician know the exact length and diameter that the stent will expand to upon deployment. A self-expanding stent which shortens in length upon radial expansion of the device can cause problems to the physician attempting to accurately position the stent within the target site. Additionally, some existing self-expanding stents can store energy axially as the outer restraining sheath is retracted. Frictional force generated as the outer sheath is retracted over the self-expanding stent can cause the stent to act somewhat like a spring, storing energy as the frictional force acts on the stent. The stored energy is released as the stent expands beyond the end of the sheath, and this release of energy can cause the stent to move or xe2x80x9cjumpxe2x80x9d from the desired position, resulting in inaccurate placement. The amount of energy stored is dependent on the flexibility of the stent and the friction between the stent and the outer sheath.
The above-described stent delivery systems also can be somewhat difficult to operate with just one hand, unless a mechanical advantage system (such as a gear mechanism) is utilized. Often, deployment with one hand is desirable since it allows the physician to use his/her other hand to support a guiding catheter which is also utilized during the procedure, allowing the physician to prevent the guiding catheter from moving during deployment of the stent. Neither of the above-described prior art stent delivery systems prevents any axial movement of the catheters of the system during stent deployment. Even a slight axial movement of the catheter assembly during deployment can cause inaccurate placement of the stent in the body lumen.
What has been needed and heretofore unavailable is a self-expanding stent which has a high degree of flexibility so that it can be advanced through tortuous passageways of the anatomy and can be expanded up to its maximum diameter with minimal, or no longitudinal contraction, and yet have sufficient mechanical strength to hold the body lumen open. The self-expanding stent should also store little or no energy during sheath retraction to prevent xe2x80x9cjumpingxe2x80x9d of the stent from occurring to allow for more accurate positioning within the body lumen. Also, there is a need for a stent delivery system which facilitates minimal movement during stent deployment, provides accurate stent placement, and provides single handed operation by the physician. The present inventions disclosed herein satisfy all of these needs.
The present invention is directed to a self-expanding stent having a configuration which permits the stent to be expanded radially to larger diameters while preventing longitudinal shortening of the stent during expansion. As a result, the present invention provides a stent which maintains a constant length from its fully compressed condition all the way through to its fully expanded condition. A self-expanding stent made in accordance with the present invention provides for more accurate placement during the delivery of the stent to the target site in the body lumen. The stent remains relatively flexible along its longitudinal axis in order to facilitate delivery through tortuous body lumens, but is strong enough radially in its expanded condition to maintain the patency of the body lumen, such as an artery or other vessel, when implanted therein.
The stent of the present invention also minimizes the potential for storing energy as the outer restraining sheath of the stent delivery catheter is retracted over the compressed stent. The structure of the stent made in accordance with the present invention stores little or no energy during deployment, reducing the likelihood that the stent will xe2x80x9cjumpxe2x80x9d off of the delivery catheter during deployment. As a result, a smooth and controlled deployment can be achieved when utilizing the stent of the present invention. This stent design results in a low profile device which maintains good flexibility to reach even distal lesions.
The stent of the present invention includes a plurality of adjacent cylindrical elements (also referred to as xe2x80x9cringsxe2x80x9d) which are independently expandable in the radial direction and arranged along a common longitudinal axis. The cylindrical elements are formed in an irregular serpentine wave pattern transverse to the longitudinal axis and continuing in a plurality of alternating peaks and valleys. Each cylindrical element is connected to an adjacent cylindrical element by at least one interconnecting member which is aligned longitudinally with another interconnecting member to create a continuous spine which runs the length of the stent to prevent any significant stent shortening during expansion. The continuous spine also helps prevent unwanted storage of energy in the stent as the outer restraining sheath of the delivery catheter is retracted to deploy the stent.
In one embodiment of the present invention, each cylindrical element is connected to an adjacent cylindrical element by three interconnecting members which are circumferentially positioned 120 degrees apart. In this embodiment, the interconnecting members are aligned to form three continuous spines along the length of the stent, again to prevent any significant shortening of the stent during radial expansion and to prevent unwanted storage of energy as the outer restraining sheath is retracted for deployment.
One preferred structure for the expandable cylindrical elements which form the stent of the present invention generally has a circumferential serpentine pattern along a plurality of alternating peaks and valleys. Each cylindrical element contains three (3) (W) and three (3) (U) shaped patterns which form the valleys of the stent. Each (W) and (U) shaped valley is connected by an (inverted U) shaped pattern which forms the peaks of the cylindrical element. As the stent expands, the (W), and (U) and inverted (inverted U) patterns open circumferentially, with the interconnecting members maintaining the spacing between each cylindrical element. To minimize the gaps between the struts when the stent is expanding, each serpentine cylindrical element is designed to extend into the space between the (W), the (U) and the (inverted U) of an adjacent cylindrical element. The interconnecting members ensure minimal longitudinal contraction during radial expansion of the stent in the body vessel. Preferably the serpentine patterns have varying degrees of curvature in the regions of the peaks and valleys and are adapted so that radial expansion of the cylindrical elements are generally uniform around their circumferences during expansion of the stent from the contracted condition to the expanded condition.
The resulting stent structure is a series of radially expandable cylindrical elements that are spaced longitudinally close enough so that small dissections in the wall of a body lumen may be pressed back into position against the luminal wall, yet does not compromise the longitudinal flexibility of the stent both when being negotiated through the body lumens in the unexpanded state and when expanded into position. The serpentine patterns allow for even expansion around the circumference by accounting for the relative differences in stress created by the radial expansion of the cylindrical elements. Each of the individual cylindrical elements may rotate slightly relative to their adjacent cylindrical elements without significant deformation, cumulatively providing a stent which is flexible along its length and longitudinal axis, but which is still very stable in the radial direction in order to resist collapse after expansion.
The stent of the present invention can be laser cut from a tube of super elastic nickel-titanium (Nitinol) whose transformation temperature is below body temperature. All of the stent diameters are cut with the same stent pattern, and the stent is expanded and heat treated to be stable at the desired final diameter. The heat treatment also controls the transformation temperature of the Nitinol such that the stent is super elastic at or below body temperature. The stent is electro-polished to obtain a smooth finish with a thin layer of titanium oxide placed on the surface. The stent is usually implanted into the target vessel which is smaller than the stent diameter so that the stent applies a force to the vessel wall to keep it open.
After the stent is expanded, some of the peaks and/or valleys may, but not necessarily, tip outwardly and embed in the vessel wall. Thus, after expansion, the stent might not have a smooth outer wall surface. Rather, they might have small projections which embed in the vessel wall and aid in retaining the stent in place in the vessel.
The elongated interconnecting members which interconnect adjacent cylindrical elements should have a transverse cross-section similar to the transverse dimensions of the undulating components of the expandable cylindrical elements. The interconnecting members may be formed in a unitary structure with the expandable cylindrical elements formed from the same intermediate product. The stent could also be made from a sheet of material with the pattern of the cylindrical elements and interconnecting elements cut by a laser. The sheet could then be formed into a cylinder by welding a longitudinal seam using laser welding or other known techniques.
Preferably, the number and location of the interconnecting members can be varied in order to develop the desired longitudinal flexibility provided by the rings in the stent structure both in the compressed condition as well as in the expanded condition. These properties are important to minimize alteration of the natural physiology of the body lumen into which the stent is implanted and to maintain the compliance of the body lumen which is internally supported by the stent. Generally, the greater the longitudinal flexibility of the stents, the easier and the more safely they can be delivered to the implantation site, especially where the implantation site is on a curved section of a body lumen, such as a coronary artery or a peripheral blood vessel, and especially saphenous veins and larger vessels. The number of spines formed by the collinear arrangement of interconnecting elements can vary from one to as many as can be reasonably placed on the stent, however, for minimal energy storage with maximum flexibility, two to four spines are preferred.
The stent of the present invention is particularly useful for implantation in body lumens which are located along the outer portions of the body where external forces could possibly be applied to the stent. For example, the stent of the present invention is particularly advantageous for implantation in the carotid arteries which are susceptible to external forces. Since the Nitinol stent is crush resistant, it will spring back to its original expanded condition even after an external force is applied to it. As a result, there is less likelihood that the stent would be deformed or crushed by an external force. Additionally, due to the springy and softer composition of the stent, there is less likelihood that the struts of the stent would cut into the underlying plaque build-up upon application of a force which may otherwise create small pieces of plaque that would enter the bloodstream.
Another embodiment of the present invention enables the stent to expand to a larger maximum size and collapse to a smaller size. The stent also has increased axial rigidity through the use of additional interconnecting members at the outermost cylindrical elements which not only increases the end strength of the stent, but may also provide additional radiopacity to the device. In this embodiment, the (W), (U) and (inverted U) portions of each cylindrical element are modified somewhat so that each arc angle xe2x80x9cwrapsxe2x80x9d more than 180 degrees, providing a smaller collapsed capacity, yet has the same size strut width. This particular embodiment of the present invention also utilizes a number of spines formed by a collinear arrangement of interconnecting elements. The number of spines can vary from one to as many as can be reasonably placed on the stent, however, for minimum energy storage with maximum flexibility, two to four spines are preferred.
The present invention also is directed to a stent delivery system which can be used to provide accurate deployment of a self-expanding stent into a target site in a patient""s body lumen. The stent delivery system in accordance with the present invention incorporates unique features which facilitate minimal movement during stent deployment, accurate stent placement, and single-handed system operation. The stent delivery system can be used to deploy the novel self-expanding stent disclosed herein, or any self-expanding stent.
One embodiment of a stent delivery system made in accordance with the present invention includes an elongated catheter body having a proximal and distal end. The elongated catheter body is made up of an inner tubular member which extends within an outer tubular member in a coaxial arrangement. The outer tubular member has a restraining sheath at its distal end which holds the stent, which is mounted on the inner tubular member, in its compressed delivery position until ready for deployment. The outer tubular member and restraining sheath are retractable to release the compressed stent to its expanded condition. The proximal ends of the inner and outer tubular members are connected to a housing assembly which provides a manual mechanism for retracting the restraining sheath and immobilizing the inner tubular member, preventing it from moving relative to the restraining sheath during stent deployment. The proximal end of the outer tubular member is attached to a pull-back handle located on the housing assembly which is moved by the physician in order to retract the restraining sheath and deploy the compressed stent. A luer fitting attached to the proximal end of the inner tubular member is rigidly fixed to the housing base to prevent the inner tubular member from moving when the outer tubular member is retracted.
The inner tubular member has a guide wire lumen which extends from the distal end of the inner tubular member to the proximal end to allow a guide wire to be used to advance the elongated catheter body to the target area in the body lumen in an xe2x80x9cover the wirexe2x80x9d technique. In this regard, the catheter stent assembly can be introduced within the patient""s vasculature in a conventional Seldinger technique through a guiding catheter. The distal end of the inner tubular member includes a soft, low profile tip assembly with a radiopaque marker. An additional radiopaque marker is placed proximal to the collapsed stent.
In one embodiment of the present invention, the inner tubular member is made with three (3) coaxial layers of materials. The inner most layer is the guide wire lumen (described above) which runs the entire length of the catheter body. A second layer of the inner tubular member is composed of a proximal portion made from stainless steel hypotube and a distal reinforcing portion which can be made from a material with high compressive strength such as polyetheretherketone (PEEK). The outermost part of the inner tubular member is a thin layer shrink tubing.
In another embodiment, the tip assembly of the inner tubular member includes a tubular element made from a piece of stainless steel hypotube to which a wound coil is welded. The coil and the distal end of the tubular element are encased in molded urethane. The distal end of the urethane body is loaded with radiopaque tungsten making the tip assembly radiopaque. The proximal end of the tubular segment can include circumferential slots which are cut into the proximal end to provide a channel which allows air and fluid to escape when the catheter assembly is flushed to evacuate air from the system.
The housing assembly of the stent delivery system is designed so that the operator retracts only the outer restraining sheath while the inner tubular member remain stationary. Due to the unique design of the housing assembly, the physician pushes down on the housing assembly during deployment and not forward. This prevents the inner tubular member assembly from moving forward toward the patient. The housing assembly includes a uniquely curved base which has a contour which conforms to the patient""s leg. The design of the housing allows the system to be operated by just one hand, freeing the physician""s other hand for other purposes, such as stabilizing the guiding catheter during stent deployment.
The stent delivery system of the present invention also includes a unique flushing system which is used to evacuate air from the system. The flushing system consists of small openings extending through the inner tubular member near the end of the proximal portion of the inner member. The openings are drilled through the guide wire lumen to effectively open up a passageway from the guide wire lumen to the annular space formed between the inner tubular member and the outer tubular member. A syringe is attached to the luer fitting at the housing assembly and sterile fluid is pumped into the guide wire lumen in order to flush air from the system. A mandrel placed in the guide wire lumen at the tip assembly blocks the flow of the sterile fluid through the distal tip. The sterile fluid is thus forced to flow out of the small openings into the annular space formed between the inner tubular member and outer tubular member. The fluid flows past the collapsed stent where the fluid will eventually escape either through the small circumferential slots cut into the tubular element of the tip assembly or from the sheath directly. Once fluid is observed dripping from the end of the restraining sheath, the mandrel can be removed since air has been evacuated from the system. Since the gap sizes are so small between the various components, capillary force prevents air from infiltrating the delivery system once the evacuation has been completed.
These and other advantages of the present invention become apparent from the following detailed description and the accompanying exemplary drawings.